1. Field of the Invention
This invention relates to a selective damper system for a structure, in particular to a selective damper system for a marine vessel.
2. Discussion of Prior Art
It is considered conventional in some areas of marine engineering to mount machinery, for example components of a propulsion unit, to a hull portion of the marine vessel through a thrust block such that thrust force generated by the propulsion unit acts on the hull portion through the thrust block to drive the marine vessel in a direction opposite to that in which the thrust force is moving.
However, because the propeller of the propulsion unit used to generate the thrust force is itself acting on turbulent water flowing around the hull of the vessel, the thrust force generated is not steady but comprises fluctuations in the magnitude of thrust force. If the frequency of these fluctuations should coincide with a structural resonance in the vessel, this resonance will be excited and will greatly amplify the thrust force fluctuations thus generating significant noise levels and subjecting the structure of the vessel to substantial stresses.
In addition the machinery in the vessel can also generate fluctuating forces and again, if the frequency of these fluctuating forces coincides with a structural resonance in the vessel, any resonance will be excited and will greatly amplify the force fluctuations, thus generating significant noise levels and subjecting the structure of the vessel to substantial stresses.
The transmission of force fluctuations through the hull of a marine vessel often means that passengers carried by the marine vessel are not able to comfortably occupy certain areas within the vessel. This means that valuable space is lost within the hull of the marine vessel.
One technique for cancelling resonance is to generate an exactly equal and opposite force, i.e. 180 degrees out of phase to the force exciting the resonance and to apply this generated force to the structure to directly cancel the excitation force so as to prevent the resonance being excited.